The invention relates generally to telecommunication systems, and more particularly to ADSL (Asymmetrical Digital Subscriber Line) transmission systems in access network.
Telephone customers are moving in an evolutionary way to higher bandwidths, from sharing POTS (Plain Old Telephone Service) and modem on one line to dedicated analog modem line at 56 kbps (kilo-bits per second) and to ISDN BA (Basic Access). Now, ADSL has been introduced and become proven technology. ADSL refers to a high-speed transmission technology using existing twisted pair of wires that permits simultaneous POTS and high-speed data communication. Generally, a higher data rate is employed downstream than upstream. Ordinary twisted pair equipped with ADSL modems can transmit movies, television, dense graphics, and very high speed data. For all its capacity, ADSL leaves POTS undisturbed. A single ADSL line therefore offers simultaneous channels for personal computers, televisions, and telephones. With ADSL, telephone companies can connect homes and businesses to exciting new interactive broadband services.
The ADSL services are different from ISDN services. ISDN provides two 64 kbps channels for voice or data, while ADSL is predominantly a data pipe providing an asymmetrical bandwidth of up to 9 Mbps downstream and 800 kbps upstream. The customers for ADSL most likely already have a dedicated modem line or ISDN.
FIG. 1 shows a conventional way of implementing ADSL over a pair gain system. A pair gain system is a system which typically includes a central office terminal (COT) located in a central office (CO) of a telephone company and a remote terminal (RT) located in a customer premises. The COT may have two or more line cards, each for connecting to a pair of wires from a telephone exchange in the CO. The COT of a pair gain system multiplexes signals received from the multiple pairs connected to its line cards, over a single pair connecting between the COT and the RT of the pair gain system. The COT also demultiplexes signals received from the RT via the single pair and provides them to the telephone exchange via the multiple pairs connected to its line cards. The RT demultiplexes signals received from the single pair and provides them to a subscriber at the customer premises over the same number of pairs. The RT also multiplexes signals received from the multiple pairs at the customer premises, over the single pair connecting to the COT. Thus, a pair gain system allows a subscriber at the customer premises to have two or more telephone lines without requiring the expense of installing additional pairs between the CO and the customer premises. An example of a pair gain system is the Miniplex.RTM. 2N1, which multiplexes two pairs over a single pair. Miniplex.RTM. 2N1 is commercially available from Raychem Corporation, Menlo Park, Calif.
As illustrated in FIG. 1, at a CO 10 of a telephone company, a telephone exchange 12 provides two analog POTS lines L1 and L2 in the form of two twisted pairs of wires to a COT 15 of a pair gain system. COT 15 multiplexes L1 and L2 over a single twisted pair 13 and converts the analog signals from L1 and L2 into DSL (Digital Subscriber Line) signals, such as ISDN signals, on pair 13. An ADSL DSLAM (DSL Access Multiplexer) 18 receives broadband data from the Internet, for example and provides high-speed data transmission in the form of ADSL signals. The DSL and ADSL signals provided respectively from COT 15 and ADSL DSLAM 18 are fed to splitter 16. Splitter 16 combines the DSL and ADSL signals for transmission to a customer premises 20 over a single twisted pair 19. Splitter 16 also receives the DSL signals and ADSL signals from splitter 32 and separates the DSL signal from the ADSL signal for transmission to COT 15 and ADSL DSLAM 18, respectively.
At customer premises 20, splitter 32 separates the DSL signal from the ADSL signal. The DSL signal is provided, via a pair 33, to a RT 34 of the pair gain system, which demultiplexes the DSL signal into analog signals on two analog POTS lines L1 and L2. Each of lines L1 and L2 at customer premises 20 is for connecting to a telecommunication device (not shown), such as a telephone or a fax machine. The ADSL signal is provided to an ADSL modem 36, which is connected to either ATM 25 (asynchronous transfer mode 25 Mbps) or Ethernet. RT 34 also multiplexes analog signals from analog lines L1 and L2 into a DSL signal over pair 33 for transmission to CO 10. Splitter 32 also receives DSL and ADSL signals from RT 34 and ADSL modem 36 respectively, and combines them for transmission to CO 10 over twisted pair 19. In the case of implementing ADSL over ISDN BA (basic access), an ISDN NT1 (network termination 1) can be used as RT 34.
One of the main limitations of ADSL is the loop length which is the distance between the CO or DSLAM and the customer premises. For service with a downstream speed of 5 to 6 Mbps (mega-bits per second), the loop length is limited to less than 12 kft, in order not to sacrifice the transmission quality. At the present time, telecom operators use very conservative installation rules to ensure that the service will always be properly delivered. However, they will soon receive requests for service from people living farther away from the central office where a DSLAM is located. At present, it is estimated that about 60% of the subscribers can be potentially reached and receive high-speed data services. This leaves the other 40% with only low-speed data services.
Of course, there are several solutions for delivering high bandwidth services over longer distances (e.g. fiber optics). One solution is the use of a repeater for the ADSL signal. However, ADSL repeaters face a specific issue with power feeding as the ADSL signal is usually transmitted simultaneously with POTS on a copper pair. In order that a repeater be practical in the field, it will need to receive remote power feeding. This is very difficult and may be impossible if ADSL is implemented with POTS on the same copper pair.
An easy solution is to run POTS and ADSL on different pairs. Several POTS lines can be multiplexed over one copper pair using subscriber line multiplexers. The copper pair used for ADSL can carry a DC current for remote power feeding an ADSL repeater. However in this case telecom operators must install two copper pairs in order to provide one or more POTS lines and an ADSL. Therefore, substantial additional expenses will be incurred by telecom operators for installing a second copper pair whenever ADSL service is requested to be delivered over longer distances.
Accordingly, there is a need for implementing ADSL over POTS over longer distances without requiring an additional copper pair for transmitting an ADSL signal along with a DC current to a customer premises.